A conventional slide fastener stringer comprises a pair of slide fastener stringer halves having a continuous helical coil or meandering coupling element attached to a longitudinal edge of a support tape and adapted to engage or disengage a complementary coupling element of another support tape. The coupling element has a plurality of turns or spirals relatively closely spaced apart along the edge of the tape and generally projecting laterally along the edge.
Typically, the coupling element has coupling heads which engage coupling heads of the complementary coupling element to prevent separation of the coupling element in the absence of a slider, which couples and decouples the elements. The coupling element may be continuous and may comprise helically coiled turns of monofilament thermoplastic joined along the longitudinal edge of the support tape. Each turn forms a coupling head receivable between the turns of the complementary coupling element.
Various techniques have been proposed for securing the coupling elements to the support tapes: 1) stitching the coupling element to the support tape with a row of chain or lock stitches, 2) clamping the coupling heads of each individual turn of the coupling element to the edge of the tape by crimping or similar techniques, and 3) thermally welding or adhesively bonding a continuous coil coupling element to the support tape.
Typically, a filler cord extends along an inner opening of the coupling element for engaging the thread that loops over the upper shank of each spiral to secure the coupling element to the support tape. However, the thread along the upper shank is exposed to slider wear and foreign object abrasion which causes the thread to break. When the thread breaks, the coupling element disengages from the support tape and the slide fastener fails.
An attempt to solve this problem utilizes an indentation along the upper shank of each turn to protect the thread from slider and foreign object wear. For example, U.S. Pat. No. 3,783,476 of Frohlich, issued Jan. 8, 1974, discloses coupling elements having an indentation for engaging the thread below the exposed surface of the coupling element's upper shank, thereby partially protecting the thread from the slider and from foreign object abrasion. However, the thread is not completely protected since it is still partially exposed along the outer surface of the coupling element's upper shank. In addition, Frohlich requires specialized coupling elements having a specific indentation, thereby increasing the complexity of the coupling elements' manufacturing process. Furthermore, the indentation must be smooth to ensure that the thread does not encounter abrasion from the indentation itself. Also, the thread must be properly centered so as to shrink within the indentation during the heat shrink process. Otherwise, a portion of the thread will remain exposed and defeat the indentation's function. In addition, the thread may be over-heated during the heat shrink process and break, causing total failure of the slide fastener.
Another attempt at a solution to the abrasion problem is disclosed in U.S. Pat. No. 3,456,306 of Heimberger, issued Jul. 22, 1969. Heimberger discloses melting the looper thread for the purpose of increasing the coupling element's ability to withstand stress. However, Heimberger is careful to point out that the looper thread is melted in "formfitting relationship" to the coupling element, i.e., is not melted completely away from the upper shank. Therefore, the looper thread remains on the upper shank, exposed to slider wear and foreign object abrasion. In addition, Heimberger states that the looper and needle threads are melted together "to form a nonseparable fastening means capable of withstanding high stress and even forming a substantially continuous member for guiding the slider". Therefore, Heimberger uses the portion of the looper and needle threads which are melted together to form a guide on which the slider travels. Thus, the Heimberger slide fastener encourages slider abrasion of the looper thread. The slider abrasion problem of Heimberger is further increased because the needle and looper thread have approximately the same tension level. Therefore, the non-melted portion of the looper thread remains in an exposed position above the filler cord after the melting process.
In summary, Heimberger teaches away from eliminating slider abrasion by disclosing the melted portion being used as a guide for the slider. Heimberger further discloses the looper thread "about" the coupling element to produce "a ridge along which the slider can ride". At least a portion of the looper thread of Heimberger remains on the upper shank of the coupling element resulting in such portion of the looper thread being exposed to increased slider wear. Therefore, the Heimberger slide fastener is not abrasion resistant and appears to be actually less capable of withstanding abrasion than a conventional slide fastener.